Biblical marriage is the union of one man to one woman. This has to do with public documents and court documents that dictate how very important processes like taxation, deed transfer, visitation, guardianship, power of attorney, and much more work.
This does not effect how ceremonial marriage works. A church is not obligated to perform a marriage. Churches do not file marriage certificates. Furthermore, many churches have performed marriages between same-sex couples since long before same sex marriage (the public law type) was legal.
To sum it up, marriage already _was_ a law. Several states had additional laws dictating what marriage was. These were at odds with the certain provisions of the US' Constitution and were impeding upon the rights of many citizens.
The "truth" here is that Marriage, as far as the Federal Government and any administrative district is concerned is simply a provision that says that Person A and Person B are entitled to certain joint rights.